1. Field of the Invention
The present invention relates to the medical industry and magnetic resonant imaging (MRI) and, more particularly, to MRI and systems, program product, and methods for acquiring data using a modified data acquisition strategy and for processing the data to simultaneously determine separate chemical species (such as water and fat) and their transverse relaxation time constants.
2. Description of Related Art
Magnetic resonance imaging (MRI) systems have become well known and well used in the field of medical diagnostics. For example, MRI is a useful tool for detecting and characterizing brain tumors and can be more sensitive and specific than other competing modalities such as X-ray computed tomography (CT) or ultrasound sonography. Over the two past decades, improvements in hardware technology and data acquisition/processing techniques for MRI examinations have permitted drastically higher-quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a primary magnetic field, a radio frequency (RF) magnetic field and time varying magnetic gradient fields with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be carefully manipulated through RF and gradient fields, detected with an RF coil, and processed to reconstruct a useful image.
Conventionally, as understood by those skilled in the art, the magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. An RF coil is employed to produce an RF field. This RF field perturbs the spin system from its equilibrium or aligned direction and causes the spins to process around the axis of their equilibrium magnetization. As nuclei of high energy state relax and realign, emitted energy can provide information about the surroundings or environment of the nuclei. The realignment along the magnetic field is known as longitudinal relaxation, and the time in milliseconds (ms) required for a certain percentage of the nuclei to realign is known as T1. In turn, the transverse relaxation refers to the process by which the transverse component of the magnetization decays to zero, and the time constant is known as T2. During this precession, free-induction decay (FID) signals are generated by the spins and are detected by either the same transmitting RF coil or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Many specific techniques have been developed to acquire magnetic resonance (MR) images for a variety of applications. One major difference among these techniques is in the way gradient pulses and RF pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios (SNR's), and resolutions. Graphically, such techniques are illustrated as “pulse sequences” in which the pulses are represented, along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of large amounts of raw data. Such pulse sequences permit significant reduction in the time required to perform the patient examinations. Time reductions are particularly important for acquiring high-resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
The most widely used data acquisition and processing techniques on modern day MRI scanners are Fourier or spin warp-based. With this approach, raw data required to reconstruct an image are collected sequentially line-by-line and placed on a rectilinear coordinate. Each line of data is referred to as frequency encoding and is collected while a frequency-encoding gradient is turned on. A phase encoding gradient of varying areas is applied before the frequency-encoding gradient to advance between different frequency encoding lines and to complete the filling of the rectilinear coordinates. After raw data are complete, simple fast Fourier transform can be applied to produce a spatial image.
The duration of the frequency encoding gradient is determined by the acquisition matrix size (Xres) along the frequency encoding direction and the receiver bandwidth, both of which are independently selected by an operator. The image resolution along the frequency encoding direction is simply determined by Xres, and the field-of-view (FOV). Typically, the higher the receiver bandwidth, the lower the signal-to-noise ratio (SNR) for the final image. On the other hand, higher receiver bandwidth leads to an image with less geometric distortion due to background field inhomogeneity and signal decay during the frequency encoding time. Another factor that needs to be considered when choosing a receiver bandwidth is that the higher the receiver bandwidth, the higher the amplitude for the frequency encoding gradient is needed. The maximum amplitude of the gradient that is available on an MRI scanner is limited and typically is within the range of 1 to 4 Gauss/cm.
Among the pulse sequences, which have been developed for fast acquisition of large amounts of MR data, is a sequence generally referred to as fast spin echo (FSE or RARE). This technique is capable of generating high-quality image data in a fraction of the time needed for conventional spin echo imaging. FSE techniques have thus become the sequence of choice, especially for T2-weighted imaging. For example, a simplified graphical diagram of an FSE pulse sequence is illustrated in prior art FIG. 2. As shown, the time available for acquiring echo signals between each 180° RF refocus pulse pair is known as echo spacing (esp). During each esp, a readout or frequency-encoding gradient pulse (gxw), having an area equal to twice the area of an initial dephasing gradient pulse (gx1), is applied to acquire echo signals. Each of the echo signals constitutes a line of the raw data in the rectilinear coordinate and is encoded differently with other phase encode gradients (not shown in FIG. 2) to form other lines of data necessary for a final image.
Perhaps a prominent and distinguishing feature of FSE images, however, is an anomalously bright signal resulting from fat content in the tissue being imaged. The phenomenon has been attributed to the demodulation of the J-coupling and de-sensitization of diffusion through inhomogeneities due to the rapidly refocusing RF pulse trains contained in the FSE pulse sequence.
Because fat often obscures lesion detection, separation of water and fat in MRI can be important. Fat suppression has therefore become desirable in T2-weighted, high-resolution imaging procedures. At present, several techniques have been employed for such fat suppression. A first such technique is referred to as chemical saturation, and can be used to reduce the fat signal, but requires very homogeneous magnetic fields due to the close separation of the water and fat signals in their resonance frequencies. In particular, the RF pulse must saturate all fat, requiring a highly uniform main magnetic field, to avoid residual fat signals. Similarly, the technique depends highly upon the homogeneity of the RF field, which is needed to achieve an accurate flip of the fat signal for suppression and subsequent flip of the resulting water signals for imaging. Inhomogeneity in the main magnetic field is particularly a problem at locations off the isocenter of the field system. Finally, patient anatomy also tends to perturb the fields, rendering the technique particularly problematic.
A second technique that has been developed for fat suppression involves short inversion time (TI) inversion recovery, and is commonly referred to as STIR. This technique is intended to flip all signals to an inverted direction, with fat and water signals recovering at different rates. The technique then acquires the image data when the fat signal is crossing the null point while the water signal is still partially in the inverted state. Because of its underlying principles, the technique typically is dependent on the T1 of the water signal and, generally, results in relatively low SNR due to the partial recovery of the water signal during the recovery of the fat signal.
A further technique that has been developed is generally referred to as the Dixon technique. In this approach, the chemical shift difference between water and fat is encoded into images with different echo shifts. Field inhomogeneity effects appear as image phase errors, which in principle can be corrected for by a combination of multipoint acquisition and more elaborate image processing. Although these techniques allow for more uniform water and fat separation in the presence of field inhomogeneity, one clear drawback is the requirement for multiple data acquisitions and, therefore, longer scan times.
Incorporating the Dixon approach with fat suppression into FSE pulse sequences presents a mutually beneficial combination. Although the Dixon technique provides a potentially robust separation of the strong fat signal, FSE helps to alleviate for long data acquisition times in the multipoint Dixon technique. In a combination of these techniques, however, echo shift as dictated by the Dixon technique is usually achieved by shifting the timing of the readout gradient and the data acquisition window to maintain necessary conditions (Carr-Purcell-Meiboom-Gill; “CPMG” conditions). As a result, inter-echo spacing can increase, leading to substantial loss in the slice coverage for a given sequence repetition time, largely offsetting the gain of using FSE for reducing the scan time. The technique is believed, therefore, to be appropriate for imaging small anatomic areas only that do not require large slice coverage.
Dixon techniques based on the conventional spin echo or gradient echo sequences generally employ shifting the echo through either shifting the RF pulse or the readout gradient/data acquisition window. In the case of FSE based Dixon techniques, the CPMG condition dictates that only the latter strategy can be used. Shifting the readout gradient/data acquisition window, however, would require increasing the echo spacing, leading to the disadvantage of longer acquisition times and less slice coverages because of the increased dead time during a sequence. Consequently, the loss of slice coverage for a given scan time, or increased scan time for a given number of slices, and an increase in image blurring and greater sensitivity to flow and motion artifacts, can all result.
Accordingly, there is a need, therefore, for enhanced techniques for obtaining shifts in echoes in MR imaging sequences. There is a particular need for an FSE-based Dixon imaging approach, which achieves the echo shifts satisfying the CPMG conditions without necessitating an increase in echo spacing. There is, at present, a particular need for technique, which can be implemented on existing hardware and control systems to obtain enhanced timing and imaging clarity in a relatively straightforward manner. Also, there is a need for data acquisition techniques that enhance fat quantitation, such as for use in diagnosis for various diseases, such as bone marrow disorders, adrenal tumors, or hepatic steatosis.
There, further, is still a need for enhanced MRI data acquisition techniques which enhance quantitation of transverse relaxation time constants to thereby be used for lesion characterization, tissue iron concentration measurement, and monitoring of progression of neurological diseases, such as Parkinson's disease. Additionally, the effective transverse relaxation (T2*) can be broken into an RF reversible (T2′) component and an RF irreversible (T2) component. Traditionally, measurement of the transverse relaxation time constants is performed by acquiring a series of gradient echoes, or more conveniently a series of FSE echoes with different echo times. These prior approaches, however, usually require very long scan times, and at most measure only one component of the transverse relaxation at a time. Also, these measurements are subject to errors in the RF flip angles or slice profiles. Still further, there is a need for enhanced MRI data acquisition techniques that reduce the time and increase the accuracy of quantitation of transverse relaxation time constants.